In a television receiver a number of techniques have been employed for transmitting the DC component of a detected video signal to the cathode ray tube (CRT) or image reproducer for establishing a black reference level. Among the alternatives available are such well known techniques as AC coupling, full DC coupling, partial DC coupling, clamping to the tip of the sync pulse signal (DC restoration) and clamping to the back porch of the horizontal blanking interval (DC restoration).
Generally, each of the above-mentioned alternatives has advantages and disadvantages. For example, AC coupling in any amount is accompanied by undesired shifting of the black reference level due to the varying amount of black in the scene or the average picture level. DC coupling is prone to undesired drifting in various stages of the amplification network. Sync tip clamping and back porch clamping as well as DC coupling cause a shift in black level due to variations in sync amplitude from station to station as well as errors in station set-up of the signals. Moreover, clamping techniques are especially susceptible to "setting-up" on undesired noise signals and require special noise immunity circuits.
In a more recent approach, the blackest video information of a viewed scene is utilized to provide the black level setting for a cathode ray tube of the television receiver. Therein, the blackest information of the "picture" portion of the video signal is used to determine the black level setting. Also, the horizontal and vertical blanking pulse signals are removed from the video signal as set forth in the above-mentioned co-pending applications. Thus, if the blackest video information in a viewed scene is held or adjusted automatically to be black, then the black level must be independent of sync pulse amplitude, back porch level, or station "set-up" levels of the remainder of the video signal.
Although the above-described technique of employing the blackest portion of the video information signal to provide a black level setting has been and still is utilized in numerous commercial television receivers, it has been found that there still remains something to be desired. Specifically, the above circuitry utilizes a sensitive peak detector system which is sensitive to noise signals. Thus, very good noise protection circuitry is required to prevent "backing off" of the signal due to random noise pulses. Also, extensive circuitry is required to insure the total elimination of the horizontal and vertical blanking pulse signals in order to prevent the sensitive peak detector from clamping thereto and ignoring the black portion of the picture information signal. Thus, extensive and expensive circuitry is undesirably required for such systems.